Fertilizers, human and animal discharge, household garbage and detergents and cleansing agents give rise to dissolved phosphate in surface waters.
Increased growth of plants, especially algae, is a result thereof. As little as 1 g of phosphorus leads to the formation of 100 g of biomass, the mineralization of which ultimately also uses 150 g of oxygen. This results in lack of oxygen so that fish will suffocate and an anaerobic rottening processes will develop; i.e., the water source "tilts over".
5-10 mg of phosphorus/liter are present in municipal sewage, the major part being o-phosphate in the form of an easily soluble sodium salt. As far as the phosphates are derived from detergents and cleansing agents, polyphosphates (e.g. sodium tri-polyphosphate) are also present. They will, however, be hydrolyzed to o-phosphates, although this will already take place in the sewage pipe.
According to the Sewage Treatment Regulation (Rahmen-Abwasser-Verarbeitungsvorschrift) from Sep. 8, 1989 (the Secretary for Environment, Protection of Wildlife and Reactor Safety) (Bundesminister for Umwelt, Naturschutz und Reaktorsicherheit) sewage from plants with more than 100,000 person units may have a maximum content of 1 mg phosphorus/liter in the effluent.
The desired value is, however, below 0.5 mg phosphorus/liter.
It is known to remove the dissolved phosphates by precipitation with dissolved metal salts.
For the removal of dissolved phosphates all metal cations which form water-insoluble or only slightly soluble phosphates can be used. Specific and preferred examples of cations are iron, aluminum and alkaline earth metals. In simplified form, the precipitation processes proceed as follows: EQU FeCl.sub.3 +Na.sub.3 PO.sub.4 .fwdarw.FePO.sub.4 .dwnarw.+3 NaCl EQU 3 CaCl.sub.2 +2 Na.sub.3 PO.sub.4 .fwdarw.Ca.sub.3 (PO.sub.4).sub.2 .dwnarw.+6 NaCl EQU 3 MgSO.sub.4 +2 Na.sub.3 PO.sub.4 .fwdarw.Mg.sub.3 (PO.sub.4).sub.2 .dwnarw.+3 Na.sub.2 SO.sub.4 EQU 2 Na[Al(OH).sub.4 ]+2 Na.sub.3 PO.sub.4 .fwdarw.2 AlPO.sub.4 .dwnarw.+8 NaOH
The pH in municipal clarification plants ranges between 7.5 and 8.2. The pH of the precipitating agent used is almost without significance because the added amounts are far too small to be able to perceptibly change pH in the total mass. For the precipitation of phosphate it is, therefore, equally possible to use strongly alkaline products (e.g. aluminates) and strongly acidic products (e.g., mixed halogenides of Al.sup.3+, Fe.sup.3+, Ca.sup.2+ and Mg.sup.2+ from the montmorrilonite chemistry, with pH&lt;1).
The selection of preferred products to be actually used depends on a number of criteria such as availability, price, the supplier's application technology service and the qualitative balancing of one precipitating agent against another.
One of the qualitative criteria is the productivity, i.e. the amount theoretically necessary to obtain as quantitative a precipitation as possible. That depends on the stoichiometric ratio between the molecular weight of phosphorus and of the metal in the precipitating agent.
For the precipitation of 1 mol of phosphorus (31 g) the theoretically necessary amounts are:
27 g Aluminum PA1 36.5 g Magnesium PA1 56 g Iron or PA1 60 g Calcium.
Another qualitative criteria is the solubility in water of the precipitated phosphate. The smaller the solubility in water, the smaller the amount of necessary precipitating agent, and the smaller will also be the remaining dissolved phosphate. Among the o-phosphates aluminum phosphate (AlPO.sub.4) has the lowest solubility in water, closely followed by iron phosphate. Among the cations mentioned above, magnesium forms the most water-soluble phosphate.
In general, the solubility of the phosphates in water increases with increasing pH.
The above-mentioned considerations form only a part of the complex quality criteria. In reality, nowadays the most often used agents are iron chloride, iron sulfate and iron chloride sulfate in the form of aqueous solutions, not only because of cost reasons and the general availability (Titanium Chemistry) but also because iron in contrast to aluminum and the alkaline earths bind hydrogen sulfide as insoluble iron sulfide and ensures sulfur-free fermentation gas. The disadvantage connected to the use of iron salts for the precipitation of phosphate is the secondary loading of the waters by soluble halogenides and sulfates.
It is true that the aluminates nowadays have a considerable share of the market, but they also have the disadvantage connected to their inability to bind the hydrogen sulfate.
Furthermore, it is known from the literature (Alan, Appleton et al, Environmental Science and Technology, Vol. 15, No. 11, p. 1383-1386, Nov. 1981 or Kawashina et al., Water Research, Vol. 20, No. 4, p. 471-475, April 1986), that iron hydroxides are usable in the precipitation of phosphate.
It has, however, not yet been proved in practical verification in large scale because the aging processes inactivates the surface of iron hydroxide and constantly diminishes the precipitation capacity of iron hydroxide. It is also not possible to work with freshly precipitated iron oxide in practice. It is necessary to ensure storage stability. Storing takes place both at the production site, in the transportation and at the consumption site. It is also not possible to completely empty the storage tanks at the production site or at the consumption site. Some parts of the product will remain and they will proceed in the aging process. The aging stability is the determining criteria for the use of iron hydroxide in the precipitation of phosphate. The patent literature offers suggestions with iron hydroxides only from certain sources and blends (DE-A-30 01 929; DE-A 32 00 164; Ep-A-0 009 718).